Getting a diagnostic test that may help detect or monitor a disease is like getting car insurance — but better. Car insurance helps you only after something goes wrong, but the right test can prevent something from going wrong before it happens.

Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

Most of us dislike the idea of tests. We are frightened of bad news. However, burying your head in the sand won't make cancer or high blood pressure go away. Not only can the right diagnostics save you considerable money by catching problems earlier, when they are often more treatable, but they can also help you live better and longer.

That doesn't mean you should rush out and demand a battery of tests from your doctor at the first sign of a sniffle. (If you do, don't be taken aback if your doc scribbles one word in your file: "hypochondriac." And then you'll be off to the shrink instead of the labs).

But there are a few tests that you'd be smart to push for.

Getting a stress test, which is designed to determine if there is adequate blood flow to your heart during increased levels of activity, is not a bad idea for men and women who are middle-aged, since heart disease is the leading cause of death in the U.S. Another diagnostic test recommended by experts to help prevent heart disease is a simple blood test that measures C-reactive protein, a marker of the level of inflammation in the blood. That's key because too much inflammation can cause an arterial plaque to suddenly rupture, triggering a massive clot and possibly even a heart attack.

Catch diabetes earlyAccording to the American Diabetes Association, there are 18.2 million Americans with diabetes, and nearly one third of them (or about 6 million people) don't even know they have it. As a major risk factor for heart disease and other disabling conditions, diabetes is a disease that requires vigilant attention and monitoring — which is why not knowing that you have it or are prone to getting it is a particularly bad thing.

Fortunately, there are two simple tests your doctor can use to determine whether you have prediabetes: the fasting plasma glucose test and oral glucose tolerance test. Both tests tell physicians how your body metabolizes glucose — and you are not necessarily doomed if the results come back with bad news. The American Diabetes Association's Diabetes Prevention Program study conclusively showed that people with prediabetes can prevent the development of type II diabetes by making changes in their diet and increasing their level of physical activity. Just 30 minutes a day of moderate exercise, coupled with a 5 percent to 10 percent reduction in body weight, produced a 58 percent reduction in diabetes.

Benefits for companiesDiagnostics tests not only save you money, but they can save your company money too. A survey of over 3,000 employers by New York-based Mercer Human Resource Consulting, a subsidiary of insurance broker Marsh & McLennan Companies, found that total health benefit cost per employee has been slowing in recent years, with only a 7.5 percent increase in 2004, compared to a 10.1 percent increase in 2003 and a 14.7 percent increase in 2002. In addition to shifting health benefit costs to employees, more and more companies are curbing health care costs by encouraging their employees to take advantage of preventive medicine. Sometimes it's more efficient to dish out cash to decrease the chances of getting sick than it is to pay for expensive procedures to fix health problems that have become full-blown.

Despite all their benefits, diagnostic tests that enable early detection of diseases are not always perfect. Prostate-specific antigen screening — a simple blood test used to detect prostate tumors — has a high rate of false-negative and false-positive results. The risks as well as the costs of further tests like biopsies present a serious clinical dilemma because not everyone who is in a high-risk category based on PSA screening actually has a high chance of developing a fast-growing, life-threatening tumor.

Dr. Leo Cheng, an assistant professor of radiology and pathology at Harvard Medical School, notes that patients whose tumors appear to be slow-growing might be better off with no treatment. That's because treating prostate cancer can cause complications, such as the inability to have an erection or control urination — not to mention the risks involved with any surgical procedure.

Understand tests' limitsUnderstanding the limitations of certain diagnostic tests often provides the impetus for researchers to improve existing methods and technologies. Cheng and his colleagues at Massachusetts General Hospital in Boston, Mass., have developed a new way of evaluating prostate cancers that may help physicians differentiate between patients who really need treatment and those who might be better off without it. Their research, which appeared in the April 15 issue of Cancer Research, uses "magnetic resonance spectroscopy" to determine which tumors are likely to spread and which are not, based on chemical profiles of the tissue.

Although there are no plans to try to turn these findings into a new screening test anytime soon, Cheng notes that as technology improves, it is entirely possible that examining the cells of prostate tissue to determine cancer risk could one day become routine.

Most of the tests on our list are covered by PPO insurance plans. If you have an HMO, and you want, say, a stress test, you generally have to get a referral from your designated primary care physician to see a cardiologist, who would then be able to order the test.